Truly, this is the best of all possible worlds.
The subject matter of Candide is among the most timeless in all literature. Why is there evil in the world? If there is a loving creator, why does he allow bad things to happen to us? If we are completely in control of our own fates, why hasn't society weeded out the dross?
Candide is a naive young man living in a peaceful kingdom. He is preparing to marry the girl of his dreams. He is cheerily oblivious to anything negative in the world, indoctrinated into the beliefs of his mentor Dr. Pangloss, beliefs that can be summarized by my lead-in quote, “Truly, this is the best of all possible worlds.”
Candide's commitment to this philosophy is tested when his kingdom is attacked. His family is violently murdered, his fiancée brutally raped and disfigured, and his mentor, Dr. Pangloss, is killed. Candide himself is sold into slavery, and things only get worse from there. His fortunes improve only in preparation for further degradation. He attempts to cling to his belief in the best of all possible worlds, but gradually grows more and more disenchanted, finally rejecting it. Oh, and somehow this book manages to be pretty funny.
The interesting thing about Candide is Voltaire doesn't actually try to directly answer any of the questions raised in it. Instead, he satirizes the blind optimism and gullibility of his countrymen, particularly the religion ones whose beliefs mirrored those of Pangloss. Candide is often presented as pure cynicism, but Voltaire's point doesn't seem to be that the world is a relentlessly terrible place as much as it is that each man makes his own way and controls his own circumstances. This is essentially the conclusion Candide himself arrives at, finally finding happiness when he decides that, both literally and metaphorically, “every man must tend his own garden.” Rather than a wholesale attack on religion and optimism, I read Candide as a call for a realistic, pro-active approach to life, where we actively make it better rather than assuming it will naturally take the best course.
Oh, and seriously, it's pretty funny.